Many patents have been granted on thermostatically controlled aquarium heaters of the type which are normally immersed in aquarium tanks for raising the temperature of aquaria to levels suitable for the maintenance and propagation of fish and other living bodies therein, and to maintain them at such levels. Prior heating devices have been responsive to changes in the temperature of the aquarium water and included bimetallic strips, interposed in the heater element circuit and carried in the heater housing atmosphere, which are deflected in accordance with changes in temperatures to make and break associated electrical contact members. Certain difficulties have been encountered in conjunction with the use of heaters of this character and one of the problems has been that the thermostatic control was basically responsive only to the temperature of the water in the tank. If such a heater was removed by the hobbyist, while connected to a power source, as sometimes happens, and placed on a relatively colder surface, for example, (so that the device continued to demand heat) the heater could easily overheat and explode. Further, there is the danger of fire and electrical shock in such situations. Moreover, localized hot spots seem inevitably to occur in varied locations on present day heater tube walls with the result that such tubes tend to crack and fail, and have a relatively short useful life.